This invention relates in general to sensors and in particular to a sensor for measuring a gas or ion concentration.
Sensors with field-effect transistors (FETs) for measuring gas concentrations, where the gate of the FET may be a gas-sensitive layer whose work function depends on an ambient gas concentration, are known for example from U.S. Pat. No. 4,411,741.
Sensors with FETs for measuring ion concentrations, where the gate of the FET may be an ion-sensitive layer whose potential depends on the ionic concentration of an ambient liquid or gas, are known for example from U.S. Pat. No. 5,911,873.
Such sensors may generally be fabricated by counter-doping a semiconductor substrate to form therein a drain and a source and growing or depositing an insulating layer on the substrate between the source and the drain. An ion-sensitive layer can be applied directly onto the insulating layer. A gas-sensitive layer can be made a certain distance away, this configuration being known as a suspended-gate FET (SGFET). Alternatively, a gate can be applied to the insulator and controlled capacitively by a gas-sensitive gate formed a certain distance away. Such a sensor, known as a capacitively controlled FET (CCFET), is described for example in German Patent DE 43 33 875 C2.
A disadvantage of these configurations may be that after a certain time, surface conductivity may pull a potential of the FET to a potential that is present on the gas-sensitive gate, causing the drain-source current to drift. To prevent this, a conductive ring, or guard ring, which can be set to a predetermined potential, may be laid around the FET. As such, the channel region of the FET may assume the potential of the guard ring after a certain time because of the surface conductivity of the region between the guard ring and the channel region. The distance between the guard ring and the channel region of the FET and the conductivity of the surface may define the time required for the channel region to take on the guard ring potential, thus establishing a relatively small concentration change per unit time that a gas signal for detection may have to be registered. This distance governs the size and the manufacturing costs of such a sensor.
What is needed is a sensor that can be manufactured at low cost, has relatively small dimensions, and achieves a relatively high degree of accuracy of measurement for a change in gas or ion concentration as a function of time, and in particular where the surface resistance between the guard ring and the FET may be made higher, so that the rise in concentration per unit time for a detectable gas signal may be increased.